A formation is penetrated by a borehole for the purpose of extracting a commodity of commercial value. Examples of such commodities include but are not limited to oils, flammable gases, tar/tar sands, various minerals, coal and water.
When considering the extraction of such materials the logging of geological formations is, as is well known, economically an extremely important activity.
Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.
It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the conditions prevailing in a region from which extraction is to take place.
This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.
The acquisition of such data typically makes use of techniques of well logging. Well logging techniques are employed throughout the mining industry, and also in particular in the oil and gas industries. The invention is of benefit in logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas.
In the logging of oil and gas fields (or indeed geological formations containing other fluids) specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation that typically is porous and that may contain a hydrocarbon-containing fluid such as oil or natural gas or (commonly) a mixture of fluids only one component of which is of commercial value.
This leads to various complications associated with determining physical and chemical attributes of the oil or gas field in question. In consequence a wide variety of well logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling.
In most cases, the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question).
A resistivity logging tool is an elongate cylinder that operates by sending an electric current from a current-emitting electrode (sometimes called a “survey” electrode) into the formation. The current spreads into the rock of the formation and returns via an arcuate path to one or more receiver electrodes located either remotely or on the tool at locations that are longitudinally spaced from the survey electrode. Return of the current is assured by setting the potentials of the return electrodes so as to promote passage of the current initially away from the emitter electrode and then back towards the appropriate return electrode.
As the current passes through the rock, the resistance of the latter, which in turn is determined by the materials of and in the rock, attenuates the current. Measuring the current and voltage at the return electrodes together with a knowledge of the geometry of the current paths provides a measure of the resistivity of the material through which the returning current has passed.
In addition to the primary current emitter electrode, a resistivity tool typically has more than one set of secondary electrodes that are located at spaced intervals along the length of the tool. In one arrangement of the electrodes, the current only penetrates a short distance into the formation, while further arrangements cause the current to flow greater distances into the formation.
The detection of the resistivity from the short penetrating electrode arrangement therefore is said to be a “shallow” measurement or log; and the resistivity measured via a further electrode arrangement is said to be, a “deep” measurement or log. As their names imply these terms are used in reference to the extent to which the current penetrates the formation before returning to the tool.
Plural resistivity measurements of various depths of penetration can be formed by combining the deep and shallow measurements with various weightings in order to determine the true resistivity Rt of the formation. From this it is possible to assess (often in conjunction with log measurements obtained using different tools) the nature of fluids captured in the formation. Inverting the value of Rt provides the conductivity of the formation, another useful parameter in log generation and analysis.
Invasion, as is well known in the art, refers to a situation in which fluid (such as drilling fluid or chemicals added during or after drilling) invades the (porous) formation surrounding the borehole. In the art, the invasion is assumed to be of “step” profile, i.e. there is assumed to be an abrupt transition from invaded to non-invaded geology. Although this is not strictly an accurate way of describing invasion, for processing purposes it is usually reckoned to be sufficiently accurate. The term “invasion diameter” is used to indicate the extent of the assumedly circular region of invasion surrounding a borehole.
The resistivity of the invaded zone is different to and often less than the resistivity of the non-invaded zone that surrounds it. This means that a resistivity log performed on the formation in the invaded zone surrounding the borehole would return a value of resistivity that is unacceptably low.
Similarly the resistivity of the borehole can significantly, adversely affect the accuracy of a resistivity measurement. The influence of the borehole is known as a “borehole effect”.
In some cases it is possible to process resistivity log information in ways that compensate for the influence of invasion and borehole effects. For example patent no GB 2458504 B, the entire content of which is incorporated herein by reference, describes a method in which two or more resistivity or conductivity logs are run in respect of the same length of a borehole, at differing depths of penetration. The logs are then compared in accordance with an algorithm in order to determine whether the resistivity values they contain tend towards a single asymptote. If so, the asymptote is taken to be an indication of Rt.
The method of GB 2458504 B represents a significant improvement in the art of resistivity logging and the determination of formation conductivity. However, it is not suitable for use in all situations.
This is partly because the need to log the same length of a borehole twice or more at differing depths of penetration may, depending on the precise type of logging tool available, call for more than one pass of logging equipment along the borehole.
Logging activity, however, can be extremely expensive, primarily because the need to log a well generally interrupts drilling and well completion activities. Since these activities can incur costs of several thousands of dollars an hour, it is generally undesirable for assets such as drilling equipment and staff such as drilling engineers to be unoccupied for any appreciable length of time.
To some extent, the cost of drilling downtime can be alleviated through the use of an array resistivity tool. In this tool type, that is known in the art, several receivers simultaneously log the same length of borehole to different depths of penetration. If problematic effects such as vertical resolution differences between the receiver electrode outputs can be overcome, such tools are often more efficient than single-measurement types.
Enhancement of the vertical resolution of array tool outputs is a major benefit of certain techniques described in GB 2458504 B; but the described method requires establishment of whether the resistivity values of the respective, differing-depth logs tend to an asymptote. Depending on the computing power available to analyse log data, this may be impossible to achieve in real time.
Moreover the techniques disclosed in GB 2458504 B require inputting of three variables, and these are not always available on a real-time measurement basis.
Real-time assessment of Rt, however, is strongly desirable for various reasons, including but not limited to the general desire to avoid drilling equipment downtime. It would therefore be beneficial to provide a technique that is useable to generate Rt to an acceptable degree of accuracy in a manner that permits real-time calculation of the quantity.